The present invention relates to a control device, which drives and controls a hydraulic actuator in accordance with the control input of an operating member in a hydraulic drive machine, such as a hydraulic excavator, or crane.
In general, hydraulic drive machines, such as construction equipment, are configured so that a drive command signal, which specifies a control input for a plurality of operating levers, is applied to a plurality of corresponding operating valves (flow control valves). The above drive command signal changes the areas of the apertures of these plurality of operating valves, thereby driving a corresponding plurality of hydraulic actuators. In other words, when a plurality of operating levers are operated simultaneously, pressure oil discharged by a hydraulic pump is supplied to a plurality of hydraulic actuators via a plurality of operating valves on a plurality of pressure oil supply channels, and these plurality of hydraulic actuators are driven simultaneously.
With this configuration, there is something called a load sensing system, which serves as the technology that cancels the so-called load dependency of the drive velocity of hydraulic actuators operated in combination.
With this system, a valve, called a pressure compensation valve, is provided either between a hydraulic pump and a flow control valve, or between a flow control valve and a hydraulic actuator, and works to compensate for the differential pressure of pressure across a valve for pressure oil, which flows through a flow control valve, so that this differential pressure is the same value for all drive shafts (In construction equipment, a drive shaft refers to a boom, arm, et cetera.). That is, in the general formula for a hydraulic circuit,
Q=cxc2x7Axc2x7(xcex94P)
(provided that Q is the flow that passes through a restrictor of a flow control valve, c is the capacity constant, A is the area of a restrictor aperture, and xcex94P is the differential pressure across a restrictor), the load sensing system works to achieve a flow Q, which is proportional to an operator-ordered drive command value (aperture area A) by making the differential pressure xcex94P the same for each drive shaft.
Further, the load sensing system works to control the discharge pressure of the hydraulic pump so that the discharge pressure of the hydraulic pump achieves a pressure whereby the above across-valve differential pressure is added to the maximum value of the load of the operating hydraulic actuators. This prevents changes in velocity (load dependency) resulting from differences in the load of each hydraulic actuator when operated in combination.
However, there are drawbacks to this system, such as complex valve configurations, and susceptibility to hunting as a result of poor hydraulic stability.
Accordingly, to solve for this problem, Japanese Patent Publication No. 6-41762 and Japanese Patent Publication No. 6-41764 propose the configuration of a system that does not utilize the above-mentioned pressure compensation valve.
That is, the points disclosed in the above-mentioned announcements make use of the above-mentioned general formula for a hydraulic circuit,
Q=cxc2x7Axc2x7(xcex94P)
and strive to find, by reverse calculations from the relational expression
A=Q/(cxc2x7(xcex94P)),
the aperture area A for achieving the target flow Q when there is a differential pressure xcex94P.
For arbitrary differential pressures xcex94P, which differ like this for each hydraulic actuator, the dependency of actuator velocity during combined operation is canceled by reverse calculating from the above general formula the aperture area required to achieve the various target flows.
Further, another method for canceling the above dependency without using a pressure compensation valve is disclosed in Japanese Patent Application Laid-Open No. 4-351304.
As disclosed in this announcement, the square root of the ratio between a differential pressure set in advance and the detected value of the differential pressure across a flow control valve is used as the correction factor to compensate for the drive command value (control input from an operating lever) for pertinent flow control valves of drive shafts other than the drive shaft with a minimum differential pressure across the flow control valve. This compensates for the drive command value so that the valve opening (aperture area) becomes as small as a drive shaft with a large across-valve differential pressure (drive shaft with a small load).
As disclosed in the above-mentioned Japanese Patent Publication No. 6-41762 and Japanese Patent Publication No. 6-41764, when finding the aperture area A, it is necessary to divide by the across-valve differential pressure xcex94P.
However, as a result of hydraulic pump flow saturation, which proves troublesome during the combined operation of hydraulic actuators, the differential pressure xcex94P between the hydraulic pump and a hydraulic actuator can often become a value in the vicinity of 0 (kg/cm2). Under these circumstances, there are times when the above-mentioned division operation is not possible.
Further, because processing requires division by a number approaching zero, the detection error of the pressure detector for detecting a differential pressure that approaches this zero impacts greatly on the accuracy of the arithmetic process, and control. Thus, to ensure better-than-constant accuracy requires a high-precision pressure detector. This is a problem in that it increases costs.
Furthermore, because the system determines whether or not it is possible to distribute the flow targeted for each hydraulic actuator from the dischargeable output of the hydraulic pump, complex processing is also required to correct target flows to each hydraulic actuator when the above-mentioned hydraulic pump saturation occurs.
Conversely, as disclosed in Japanese Patent Application Laid-Open No. 4-351304, similar to the above, when an across-valve differential pressure xcex94P is corrected, since this corrected differential pressure becomes the denominator, to avoid having zero (near zero) as the denominator, the system is designed to perform control so that this correction processing is not carried out for a drive shaft with a minimum across-valve differential pressure xcex94P.
However, as a result of this, there is the problem that when switchover to the drive shaft with the minimum across-valve differential pressure xcex94P takes place, it is necessary to switch from correction-based control to ordinary control. In that instant when switchover occurs, the continuity of drive command values to hydraulic actuators is interrupted, generating a shock each time switchover occurs.
Furthermore, as disclosed in this announcement, with regard to hydraulic pump control, what is called pump load sensing control is essential so that the differential pressure between the discharge pressure of the hydraulic pump and the maximum load pressure of the plurality of hydraulic actuators (minimum differential pressure) becomes the differential pressure that was set in advance. The idea is to digitize a conventional load sensing system as-is, and to deal with a small differential pressure by shutting off control.
The invention of the present application takes these facts into consideration, and has as a first object to make do with a simple hydraulic circuit, which does not utilize a pressure compensation valve, and to enable the use of an inexpensive, low precision pressure detector, to enable the maintenance of continuous control at all times using simple controls, and to get by without shocking the operator or the mechanical parts, and to cancel the load dependency of a hydraulic actuator flow during combined operation without limiting the control system of the hydraulic pump.
Further, the hydraulic actuator pressure compensation characteristics of all of the above-described prior art are univocally established, and these pressure compensation characteristics cannot be changed according to circumstances. As a result, prior art is unable to satisfy the following requirements.
That is, if we consider work that requires pressure compensation, and work that does not require pressure compensation, for work that requires fine lever control capabilities, such as suspension work, normal surface adjustment work, and finishing work, because the load dependency of the flow (actuator velocity) greatly impacts work efficiency, pressure compensation is required often.
Conversely, when performing release work following excavation, and when moving the cutting edge of the bucket to the next excavation site after dumping the arm, an operator prefers xe2x80x9cload-bearingxe2x80x9d movement during rough, full-lever operations.
When pressure compensation is constantly applied even during full-lever operations such as these, if the operating lever of a high load shaft is operated even slightly, the discharge pressure of the hydraulic pump rises abruptly at that moment, the dischargeable flow determined by the equivalent horsepower performance of the hydraulic pump drops, and the flow to other drive shafts also increases. This gives rise to the problem of the velocity of the hydraulic actuator on the light load side decreasing more than necessary.
During this kind of full-lever operation, the speed of the light load working machine is required more than split control in accordance with the control input of the operating lever. What is required is believed to be a xe2x80x9cload-bearingxe2x80x9d split, that is, control that weakens pressure compensation.
Further, an operation performed frequently during hydraulic excavator work is xe2x80x9crough-combingxe2x80x9d whereby the surface of the ground is leveled. During this operation as well, an operator prefers to move the cutting edge of the bucket roughly horizontally over the surface of the ground using a full-lever boom-up, arm-excavation operation. If there is no pressure compensation at this point, because there is little pressure for raising the boom, the cutting edge of the bucket moves roughly horizontally over the ground. But when all-out pressure compensation is applied, it gives rise to the problem of the trajectory of the cutting edge of the bucket rising high into a circular arc.
In other words, when the pressure compensation function is put to use univocally, during fine control operations, combined operation can be readily performed using the operating levers without need for concern. However, there is a problem during combined full-lever operations in that speedy, xe2x80x9cload-bearingxe2x80x9d work cannot be carried out using conventional rough operations.
The present invention takes this fact into consideration, and has as a second object enhancing lever controllability, and improving work efficiency by making it possible to change pressure compensation characteristics of a hydraulic actuator in accordance with operating lever operating status and load pressure.
To achieve the above-mentioned first object, a first invention of the present invention is a control device for a hydraulic drive machine which comprises a hydraulic pump, a plurality of hydraulic actuators provided in correspondence with a plurality of operating members, and a plurality of operating valves for supplying to corresponding hydraulic actuators a pressure oil discharged from the hydraulic pump at a flow rate that accords with control inputs of the operating members, and which drives the hydraulic actuators in accordance with the operation of the operating members, characterized in that the control device comprises:
differential pressure detection means for detecting for each operating valve a differential pressure between a pressure of the pressure oil flowing into the operating valve and a pressure of the pressure oil flowing out of the operating valve;
minimum differential pressure selection means for selecting a minimum differential pressure from among the differential pressures detected by the differential pressure detection means;
correction factor calculation means for calculating for each operating member a correction factor for correcting, on the basis of a ratio between the detected differential pressure of the operating valve and the selected minimum differential pressure, a control input of an operating member corresponding to the operating valve; and
control input correction means for correcting the control input of a corresponding operating member using the correction factor calculated by the correction factor calculation means.
That is, in accordance with this configuration, a correction factor for correcting the control input of an operating member that corresponds to this operating valve is calculated for each operating member on the basis of the ratio between the detected differential pressure across an operating valve and a minimum differential pressure selected from among the differential pressures across a plurality of operating valves. And the control input of a corresponding operating member is corrected using this calculated correction factor.
Consequently, .the larger the across-valve differential pressure, that is, the smaller the load on a hydraulic actuator, the greater the reduction of control input, which is the drive command value related to that operating valve, and the smaller the aperture area of that operating valve. Thus, the lighter the load on a hydraulic actuator, the more control is applied to the large incoming flow to that hydraulic actuator. This enables the distribution of flow to each hydraulic actuator during combined operation to be in accord with the ratio of the control input of each operating member manipulated by an operator.
More specifically, using the above-described general formula for a hydraulic circuit Qi=cxc2x7Aixc2x7(xcex94Pi) (provided that i is the ith hydraulic actuator, operating valve or operating lever), the control input (drive command value), which specifies the aperture area (Ai), is corrected using a correction factor Ki=(xcex94Pmin/xcex94Pi). Therefore, the flow Qi, which flows through an operating valve i and is supplied to a hydraulic actuator i becomes Qi=cxc2x7Aixc2x7Kixc2x7(xcex94Pi)=cxc2x7Aixc2x7(xcex94Pmin/xcex94Pi)xc2x7(xcex94Pi)=cxc2x7Aixc2x7(xcex94Pmin), wherein the xcex94Pi nullify one another. Also, here xcex94pmin stands for minimum differential pressure.
That is, by having as a criteria a common minimum differential pressure xcex94Pmin for all hydraulic actuators, the flow Qi supplied to the ith hydraulic actuator is determined solely by the size of the aperture area command value Ai.
At this point, if the invention of this application is compared with prior art, which utilizes a pressure compensation valve, it is evident that with the invention of this application, hydraulic actuator load dependency during combined operations is canceled without utilizing a pressure compensation valve as in the past. In other words, a simple hydraulic circuit is used without a pressure compensation valve, and costs are reduced.
Further, if we compare the present invention with the prior art disclosed in the above-mentioned Japanese Patent Publication No. 6-41762 and Japanese Patent Publication No. 6-41764, or the invention disclosed in the above-mentioned Japanese Patent Application Laid-Open No. 4-351304, with these prior art, it is necessary to perform a division operation having a differential pressure value xcex94P of zero or near zero as the denominator. In this division operation, the values of the denominator and numerator are not necessarily the same, and the value of the numerator is a considerably larger value than that of the denominator. Consequently, the above-mentioned division operation result in xe2x80x9czero divisionxe2x80x9d and overflow, giving rise to a situation wherein arithmetic processing and control are not possible. By contrast, with the invention of the present application, because a configuration is used whereby the minimum differential pressure is divided by the differential pressure xcex94P detected at each operating valve, the denominator, which is the detected differential pressure, will always be larger than or equal to the numerator, which is the minimum differential pressure. Therefore, the xe2x80x9czero divisionxe2x80x9d and overflow of the above-mentioned division process does not occur, and there can be no circumstances under which arithmetic processing and control become impossible.
Moreover, when division is performed using a number near zero (when the detected differential pressure is the minimum differential pressure), the numerator is also the minimum differential pressure, so the quotient is always 1, and errors contained in a detection error offset one another. Conversely, even when division is performed using a detected differential pressure other than the minimum differential pressure, because the detected differential pressure denominator is larger than the minimum differential pressure numerator, the process is completed with little impact from detection errors. That is, because better-than-constant precision can be ensured even using an inexpensive, low-precision pressure detector, cost reducing effects can be achieved.
Moreover, with the invention of this application, control is performed using a compensation calculation that divides the minimum differential pressure by the detected pressure xcex94P at each operating valve without having to switch from compensation calculation-based control to ordinary control each time there is a switchover to a drive shaft with the minimum across-valve differential pressure. As a result, the drive command values to a hydraulic actuator are continuous during the instant of control switchover, thus solving for the past problem of a shock being generated at each switchover. This achieves the effect whereby control continuity can be maintained via simple control, and the operator and machine parts are not shocked. Moreover, in accordance with the invention of this application, there are no limitations whatsoever placed on the hydraulic pump control system.
Further, to achieve the above-mentioned second object, a second invention of the present invention is a control device for a hydraulic drive machine which comprises a hydraulic pump, a plurality of hydraulic actuators provided in correspondence with a plurality of operating members, and a plurality of operating valves for supplying to corresponding hydraulic actuators a pressure oil discharged from the hydraulic pump at a flow rate that accords with control inputs of the operating members, and which drives the hydraulic actuators in accordance with the operation of the operating members, characterized in that the control device comprises:
differential pressure detection means for detecting for each operating valve a differential pressure between a pressure of the pressure oil flowing into the operating valve and a pressure of the pressure oil flowing out of the operating valve;
correction factor calculation means for calculating for each operating member a correction factor for correcting the control input of the operating member in accordance with the differential pressure detected by the differential pressure detection means;
setting means for setting for each operating member an upper limit value or a lower limit value of the correction factor in accordance with the control input of the operating member; and
control input correction means for correcting the control input of a corresponding operating member, using a correction factor of which upper limit or lower limit is limited, so that the correction factor calculated by the correction factor calculation means does not exceed the set upper limit value or lower limit value.
That is, in accordance with this configuration, an upper limit value or lower limit value of the correction factor is set for each operating member in accordance with the control input of the operating member, making it so that a correction factor obtained in accordance with a detected differential pressure does not exceed the above-mentioned set upper limit value of lower limit value, and the control input of a corresponding operating member is corrected using a correction factor, which is limited by this upper limit or lower limit.
As a result of this, when the control input for a operating member (operating lever) is small, as during fine control operation, for example, the pressure compensation function is given full play without the correction factor being limited by an upper limit value or lower limit value. Further, as lever control input becomes larger, the upper limit or lower limit of a correction factor is further limited by an upper limit value or lower limit value, and the pressure compensation function becomes more moderate.
Consequently, the larger the across-valve differential pressure during fine control operation, that is, the smaller the load on a hydraulic actuator, the greater the reduction of control input, which is the drive command value related to that operating valve, and the smaller the aperture area of that operating valve. Thus, the lighter the load on a hydraulic actuator, the more control is applied to the large incoming flow to that hydraulic actuator. This enables the distribution of flow to each hydraulic actuator during combined operation to be in accord with the ratio of the control input of each operating member manipulated by an operator, improves control capabilities during fine control operation, and, in turn, enhances work efficiency.
Conversely, during full-lever operation, unlike at fine control operations, the pressure compensation function is weakened, and the large incoming flow to a hydraulic actuator with a light load is not controlled. In other words, at full-lever operation, since light-load working machine speed is required more than split control in accordance with the control input of an operating lever, by implementing control geared to this requirement, the present invention improves controllability at full-lever operation, and work efficiency is enhanced pursuant thereto.
Further, the above-mentioned limitations placed on the correction factor are not applied in accordance with the lever operating state alone, but rather can also be established in accordance with the load of the working machine. This affords an operator optimum pressure compensation characteristics for the job at hand, enables good control of an operating lever at all times, and dramatically improves work efficiency.